1. Field of the Invention
The present invention relates to an optical scanning apparatus which makes a light beam scan on a surface-to-be-scanned in a main scanning direction and an image forming apparatus which forms an electrostatic image using such an optical scanning apparatus.
2. Description of the Related Art
An apparatus which uses this type of optical scanning apparatus is an image forming apparatus such as a laser printer, a copier machine and a facsimile machine. For instance, in the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2003-43393, a laser beam modulated in accordance with image data impinges upon a deflector via a collimator lens, a cylindrical lens and an aperture stop and is hence deflected. The specific structure is as follows.
In this optical scanning apparatus, the laser beam emitted from a semiconductor laser passes through the collimator lens and the cylindrical lens and is accordingly shaped so that the cross sectional shape of the laser beam becomes an elliptic shape which elongates in the main scanning direction. Within a sub-scanning cross sectional surface, this laser beam impinges upon a deflection mirror surface of the deflector at an angle in a sub scanning direction, that is, the laser beam makes an oblique incidence upon the deflection mirror surface. In addition, this apparatus uses a galvanometer mirror as the deflector which reciprocally vibrates about a central axis of vibrations in the main scanning direction within a scanning range between maximum amplitudes ±θ max (deflection angle θ: −θ max to 0° to +θ max), deflecting the laser beam which is incident upon the deflection mirror surface. The laser beam deflected in this manner forms an image through a corrected lens on a photosensitive drum surface (surface-to-be-scanned). An electrostatic image which reflects image data is thus formed on a photosensitive drum (which corresponds to the “latent image carrier” of the present invention).